5.1 Biology of Phaeosphaeria nodorum
Knowledge of the biology and epidemiology of a fungal disease contributes to the choice of management practices to prevent major disease outbreaks in a crop. One parameter to recognise is how the fungi are dispersed. In the case of the Swedish P. nodorum population the knowledge about the sexual part of the fungal life cycle has been confirmed (II). P. nodorum reproduces through both asexual and sexual reproduction. A strong indication of sexual reproduction is that the mating type distribution is in a 1:1 ratio, and thereby the prerequisites for mating are present. The genetic variation within fields was found to be diverse with many unique genotypes. The alleles of the loci were not linked to each other, which is consistent with frequent recombination occurring during sexual reproduction. On the other hand, it cannot be excluded that the population structure is influenced by asexual reproduction, since isolates with identical genotype were found. All of these may not be clones after all since identical genotypes could occur by chance.
The population size of P. nodorum has most likely fluctuated during the long period of domestication of wheat. The genetic study (II) revealed that the Swedish population of P. nodorum has most likely experienced at least one bottleneck, a drastic decrease in population size, some time in the past.
It might have begun already in the Fertile Crescent where the ancestors of today’s T. aestivum were first domesticated and the interaction between fungi and host began (Salamini et al., 2002; Zohary & Hopf, 2000). When wheat cropping spread out of the Near East, the number of fungal strains that followed the seed to other areas or continents might have been limited. It is
Another suggestion is that the population structure changed when wheat was introduced to Sweden since the P. nodorum population is different from the Swiss and Danish populations studied earlier (Stukenbrock et al., 2006).
There is also a possibility that the harsher Swedish winters decrease the survival of the fungus on straw debris.
The presence of the sexual part of the life cycle of P. nodorum may have large consequences for agriculture. Ascospores are easily dispersed by wind, and favourable genetic and phenotypic characteristics can be spread effectively (Bathgate & Loughman, 2001). Various individuals of P. nodorum have different abilities to infect wheat and to produce progeny (III). If the ascospore is a product of mating between two aggressive isolates, the strain developed from the ascospore is most likely also aggressive. With increased aggressiveness the fungal strain produces more progeny (conidia or ascospores) which in their turn are dispersed further in the crop or to other fields. The impact of the aggressiveness of the fungal strain is also determined by the susceptibility of the wheat cultivar. Even though much effort has been made from the breeding companies to develop less susceptible cultivars, the wheat cultivars commonly grown in Sweden today are susceptible to P. nodorum both at the seedling stage (III) and as adults grown under field conditions (Larsson et al., 2006).
It has been discussed whether seedlings are more susceptible than adult plants (vide 3.5). Plant breeding should involve screening of disease susceptibility on seedlings, plants or detached leaves, and adult plants in field trials. The technique with detached seedling leaves inoculated by spraying a spore suspension is a simple and time saving way of screening breeding material for less susceptible lines. Informative screenings could be performed with low inoculum concentration to simulate natural infection, even though the disease severity increased with increased inoculum concentration (III).
The reproducibility of the technique should however be investigated further, since the variability of the isolates and cultivars was demonstrated during the repeated test of aggressiveness and host susceptibility. If the technique is standardised and adjusted for plant material, sporulation of isolates and inoculum concentration, the method would be efficient in determination of the susceptibility of cultivars.
5.2 Fungal community on necrotic wheat leaves
The composition of the fungal community on the leaves is influenced by management practises, cultivar susceptibility and dispersal of spores from various fungal species. Besides fungicides, environmental conditions such as
humidity, temperature and nutrient supply are important parameters that determine the growth of the fungi on the leaves (Blakeman, 1985). The community consists of yeasts and saprotrophic fungi together with necrotrophic pathogens (I). The abundance of yeast and saprotrophs are similar within fields and across years and regions, but with some variation in species structure. Pathogenic fungi often co-occur even though one seems to dominate. During the third year of sampling (2005) the dominating pathogen in the field was another species than previous years, which may be correlated to the weather conditions with drier spring and more rainfall in June than in an average year (SJV, 2005a; SJV, 2005b).
Data obtained from a long term field inventory at Broadbalk, u.k., has shown a shift in occurrence of two of the most common pathogenic fungi, P. nodorum and M. graminicola, while P. tritici-repentis is not a problem in the u.k. (Shaw et al., 2008). The pathogen abundance was found to be correlated to weather data in a short-term perspective where rain during spring increased the infections of both pathogens whereas P. nodorum was more abundant during warm summers. In a long term perspective the abundance of P. nodorum fluctuated from 1844 to 2002, with a peak during approximately 1960 to 1985. The peak coincides with the increased amount of sulphur emission from the use of fossil fuel, indicating that sulphur may be favourable for P. nodorum. M. graminicola dominated at Broadbalk during the last years of the twentieth century and the first years of the twenty-first century, while the frequency of the fungus was low during the rest of the twentieth century (Shaw et al., 2008). Similar patterns of pathogen abundance have also been observed in Sweden, where we have seen a decline in P. nodorum, but not as drastic as the one in the u.k. (personal observation and communication with farmers and the Plant Protection Centres). M. graminicola and P. tritici-repentis have increased during the past years and can lead to severe yield reduction. However, the visual estimations of the abundance of P. nodorum and P. tritici-repentis may be delusive since the symptoms of tan spot and stagonospora nodorum blotch are hard to distinguish (vide 3.3). It is also difficult to see the P. nodorum pycnidia in field situations, even with a hand lens. The deposition of sulphur has decreased in Sweden during the last decades (Hultberg & Ferm, 2004) and it is possible that the change of fungal community was affected by the change in sulphur deposition in Sweden as well, but that is yet to be analysed.
5.3 Fungicide sensitivity of P. nodorum
Mutations of nucleotides that lead to reduced sensitivity to fungicides will be selected if the fungal population is treated with fungicides. One clear example is the substitution g143a in the gene encoding cytochrome b, abundant within the Swedish population of P. nodorum (IV). Since this is the first report on the sensitivity of P. nodorum to strobilurins, one can only speculate when and where that substitution occurred. It must have happened soon after the release of the strobilurins in the late 1990s since the substitution was frequent already in 2003 in samples from an untreated plot of the field. Strains with the substitutions have been selected by strobilurin treatments of the crop the previous year. The strains which had lost their sensitivity could therefore have overwintered in the stubble or originate from incoming ascospores carrying the substitution.
Based on the genetic variation in the mitochondrial dna the g143a substitution occurring within the European populations of M. graminicola is thought to originate from four independent mutation occasions (Torriani et al., in press). The substitution has then most likely migrated within Europe in a west-to-east direction through ascospore dispersal which founded new insensitive subpopulations. The dispersal pattern of the g143a substitution within the population of P. nodorum is still unknown but follows presumably the dispersal pattern of ascospores, conidia and harvest residues. Another speculation of the origin of the g143a substitution found in the P. nodorum population is that horizontal gene transfer occurred from another fungal species. P. nodorum may also have been the source of g143a substitutions found in other species. This theory is based on the suggestions that the gene encoding ToxA in P. tritici-repentis was transferred from P. nodorum due to the large resemblance to the toxin in P. nodorum (Friesen et al., 2006; Knell, 2006). Friesen et al, (2008a) suggest that horizontal gene transfer is more common between species within the order Pleosporales, which contains both P. nodorum and P. tritici-repentis. The g143a substitution may therefore have been exchanged between them. The substitution in P. tritici-repentis was discovered prior to the one in P. nodorum but this has little bearing on which one was the donor and which one was the receipient.
The loss of sensitivity against strobilurins due to the g143a substitution within the population of P. nodorum, combined with the abundance of P.
nodorum found within the fungal community, shows that this fungus is still an important issue for research and wheat growers. The fungus is present in the fields but due to the resemblance of stagonospora nodorum blotch to tan spot, we can underestimate the amount of either pathogen. This means that we have to consider the risk of reduced strobilurin efficacy in inhibition of
all pathogens in all wheat growing areas in Sweden, not only the southern part of Sweden where reduced sensitivity of M. graminicola and P. tritici-repentis already has become an important problem (Almquist et al., 2008;
Jørgensen, 2007).
The results from the fungicide sensitivity test (IV) revealed that control of P. nodorum can still efficiently be achieved by using fungicides based on triazoles and anilinopyrimidine. Use of strobilurins is not advisable for crop protection due to the high frequency of the g143a substitution. Crop rotation and proper soil management are the most important ways of preventing fungal infection, by decreasing the amount of inoculum produced in the field. The crop cannot be protected from incoming ascospores of various species or airborne conidia that can start epidemics in the field.
The future of the fungicides is uncertain. The fungicides that are effective today may have lost their efficacy due to fungal evolution or they have been removed from the market in the future. Much depends on the new directives from the Commission of the European Union on the legalisation and usage of pesticides that are under consideration. The Swedish Board of Agriculture has written a report of the interpretations from the Swedish Chemical Agency (KemI) and the British Pesticides Safety Directorate (psd) (Eriksson et al., 2008). The interpretation made by psd predicts a stronger effect on wheat production than the Swedish interpretation. According to psd propiconazole may fall for the cut off criteria and cyprodinil will be substituted with another substance. Strobilurins may still be allowed for treatment against rust fungi. In practice, farmers are advised to use strobilurins in combination with triazoles against rusts (G. Berg, Plant Protection Centre, personal communication). Treatment of leaf spot diseases with strobilurins is not advisable due to the reduced efficacy against M.
graminicola and P. tritici-repentis. Now we know that the Swedish populations of P. nodorum also have lost their sensitivity to strobilurins (IV) and farmers must therefore rely on the triazoles for controlling leaf spot diseases in wheat. The alternatives are limited since epoxiconazole, commonly used in fungicide testing, is not approved in Sweden and propiconazole has declined in usage during the last years due to use of other fungicides. This leads to increased reliance on those triazoles that exists on the market, of which prothioconazole dominated in 2008 (G. Berg, Plant Protection Centre, personal communication).
In an environmental perspective there are a few biological products for reduction of pathogens (vide 3.6) and research is ongoing for more and
price of wheat and fuel are also aspects that influence the farmers’ choice of plant disease management methods and strategies (Söderberg, 2008).
5.4 Final reflection
In summary, sexual reproduction and ascospores are of importance for the epidemiology of P. nodorum in wheat. This can lead to dire consequences for agricultural practises including the prevention of disease since epidemics can occur anywhere under favourable weather conditions if ascospores manage to start an epidemic. The fungal population founded by ascospores may carry the substitution that causes reduced sensitivity to strobilurins, which is another threat to modern agriculture. Breeding for resistant cultivars, with help of the knowledge of the toxin-receptor interaction would be an important first step. The use of them, together with good agricultural management practices, including a crop rotation that reduces the presence of inoculum, would be one way of reducing the incidence of stagonospora nodorum blotch.
This thesis began with the concept of “eat or be eaten”. There are many species that take their nourishment from wheat, including us human beings.
It is the human fight against pathogens for higher yield for bread baking, and this competition also takes place with saprotrophs and yeasts. The fungal species have their internal fight against each other. These interactions could be used in biological control in purpose to reduce the environmental impact caused by high treatments by chemical substances and increased fuel use due to more intensive soil management. Much can be learned from the ecology of the system and this can be used in plant pathology and elsewhere. Plant pathology will continue to be an important subject of research, since all species need nutrients for reproduction for their survival and it will always be a give and take, eat or be eaten.